Telematic services and systems, such as OnStar™, allow customers to communicate with a central office in the event of an emergency, from within a vehicle. The telematic systems can initiate communication with the central office and relay vehicle and occupant status information without user intervention. Currently, telematic service communication is provided by terrestrial cellular service providers, which have limited geographic coverage. There is a desire for telematic systems to use satellite communications in order to provide wider geographic availability.
Unfortunately, satellite communication can be hindered or obstructed by interference objects formed of dense materials having low signal transmissive properties. For example, when a vehicle is located within a parking structure, concrete floors of the parking structure can reduce signal power between the vehicle and a satellite. Each interference object, such as each floor of the parking structure that is inbetween the vehicle and the satellite can reduce signal power by 10 dB, or a factor of 10 or more.
Transmission power of a satellite is larger than that of user terminals or cellular communication devices within a vehicle. The reduction in signal power, due to interference objects, can cause attenuation of transmitted uplink signals from the vehicle to such an extent as to prevent communication with the satellite.
It has been suggested that in order to increase signal penetration of interfering obstructions, that transmission power levels be increased and that data transmission rates be decreased. Transmission power levels may to some extent be increased on a satellite but increased power is unlikely on a user terminal, due to physical and cost restraints. However, both uplink and downlink communication can be improved by reducing the data rate and increasing the duration of the transmission. For example, a signal transmitted at a data rate of 4 bits/sec has approximately a 30 db advantage over a signal transmitted at a data rate above 4 Kbits/sec.
Satellite communication networks and terrestrial wireless networks have complementary strengths and weaknesses. Satellite networks cover wide geographical areas, while terrestrial networks are limited to areas that are within a range of a set of base stations. On the other hand, terrestrial networks are better capable of serving large populated user areas than are satellites. To provide for heavy concentrations of users, within a terrestrial network, additional base stations are utilized within heavily populated user areas. In order to benefit from strengths of both the satellite networks and the terrestrial networks, sharing communication frequency bands between satellite networks and terrestrial networks has been suggested. Unfortunately, to share communication bands typically requires complex and extensive coordination between satellite and terrestrial networks.
Another disadvantage of using a satellite network over an urban area, for which a terrestrial network is commonly used, is that obstruction of communication signals commonly occurs due to buildings and other objects. Terrestrial base stations are inherently better for receiving communication signals from a user terminal than a spacecraft, due to angles of transmission. For example, when a user terminal is within a parking structure, in communicating with a satellite, transmission angle is approximately between 45°–90°. When the same user terminal is in communication with a base station, transmission angle is typically less than 5°, allowing the signals to be transmitted through fewer dense obstructions, such as concrete floors of a parking structure.
Although the obstructions may be fewer in the horizontal direction, certain features of cellular air interface design limit communication with the terrestrial base stations. Power levels of base station transmissions are limited in order to reduce interference with nearby cellular terminals, which use the same frequency. Terrestrial wireless communication standards do not support communication when the user terminal is unable to sufficiently detect a transmitted signal from the base station. Therefore, when base station normal transmission power is insufficient to penetrate a parking structure, cellular-based communication cannot occur. Cellular air interface standards do not permit ad hoc increase of base station transmitting power to overcome signal path losses and increases in power by a user terminal to reach the base station. Further, cellular air interface standards do not support communication at data rates less than approximately 4 Kbits/sec, net after error correction coding. Ultra-low data rates improve transmission of an emergency signal through obstructions. An emergency signal only requires transmission of a few bits per second.
It would therefore be desirable to provide a larger geographical area coverage telematic system that provides communication capability with user terminals, especially during emergency situations when interference objects are present, as described above.